Massage sayonara USES

• when gstreamer1 plugins are missing, files just don't play (this applies also to, say, having a broken ogg plugin).
• complete the list of useful gstreamer plugins; while typical users will have -core installed, and thus -libav, make dependency on libav explicit. This plugin is used for m4a, aac, wma and mpc files.
• drop USES=gettext-runtime, which I added in r451527. It is pulled into LIB_DEPENDS by USE_GNOME=glib20, so it is a duplicate here. Thanks to danfe@ for explanation.

Reviewed by: danfe
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D12629